1. Field of the Invention
The disclosure concerns a process and a device for compensating for, or correcting, measurement errors, and/or signal shaping of analog measurement signals, especially for sensors such as piezo-resistive sensors, in which a transformation of analog measurement signals into digital values is undertaken.
2. Related Prior Art
Sensors, to include also piezo-resistive sensors, have been completely integrated into modern technology and form an indispensable component for a measurement, control and/or regulatory circuit. By their use, precise measurement values can be obtained and then be further processed.
In contrast to previous decades, modern sensors are available on the market at relatively favorable prices.
In practice, the problem certainly arises that, for example, the measurement signal picked off at a measurement bridge can be contaminated with many measurement errors. Further, traditional sensors can be employed in association with microprocessors only by means of expensive switching, as a result of which, in any case, the measurement signal obtained from the analog measurement circuit must first of all be subjected to an analog-digital transformation.
In order to at least permit a temperature compensation, U.S. Pat. No. 4,192,005 discloses that a measurement value compensation switch can be provided downstream from the measurement point, in order to at least compensate for the errors arising from the influence of temperature in the analog measurement signal. But even this analog temperature compensation is imperfect, based on results.
On the other hand, U.S. Pat. No. 4,192,005 discloses how to digitize an analog signal measurement signal of a piezo-resistive sensor and process it digitally, through an analog-digital converter. This leads to an extreme loss of analog-digital converter resolution, since the sensitivity and offset compensation cannot be set from the start. In other words, with this known device, the entire measurement signal, including all errors, will be digitized from the outset. In this situation, the resolution can, at the maximum, openly reflect the individual digital steps. In addition, an enormous fabrication expense and, above all, a multiplicity of storage registers is required in the downstream digital processing electronics, in order to at least begin, at the outset, to process the original analog measurement signal after its transformation in the analog-digital converter, so that at least a part of the measurement errors will be compensated from the outset. Since the fabrication cost is enormous, the use of a downstream microprocessor was recommended in this connection in the U.S. patent document mentioned above.
By comparison to the state of the art just discussed, a greatly enhanced system for measurement error compensation has been made known in EP-A 0 169 414. The measurement processing and the corresponding device set forth in this document for the accomplishment of measurement processing includes a compensation circuit working in digitized form. The measurement signals, received in analog form, are further processed in analog form in the course of which an analog measurement signal is digitized only in a regulatory circuit, in order to then call up a corresponding prestored compensation or signal-shaping value based on a calibration and feed it back into the measurement switch. By this means, for example, the power supply for the measurement bridge and/or the amplification curve in the operational amplifier connected downstream from the measurement bridge can be influenced. With the state of the art just discussed, on the other hand, far better results can be achieved than by earlier methods. Particularly, by this approach, not only can a temperature compensation be provided but, above all, a zero pint and linearity compensation of the received measurement signal can also be undertaken.
With improved switching, there are also disadvantages to the state of the art which lie in the areas of long-term instability and a relatively high energy consumption. Analog compensation with an operational amplifier necessitates relatively large power signals in the mA [milli-Amp]range, since otherwise the output signal of the operational amplifier is submerged by noise. Finally, the analog initial signal is not suited for direct further processing by means of a microcomputer, and therefore in this case an additional analog-digital converter must be provided. Finally, the expense of the calibration display is also worth consideration, since many individual calibration steps are necessary, in order to be able to execute a desired measurement error compensation.
Although the state of the art has proved itself with greatly enhanced compensation and signal-processing, in contrast to earlier solutions, disadvantages nonetheless remain.
In essence, there are the following sensor signal measurement errors which should be considered in an optimal combination process.
a) A possible measurement error concerns the "null-point displacement and scattering of the measurement value," which is caused by varying resistance values in the two arms of the measurement bridge, so that even without pressure being applied to the pressure sensor, a null-point error pressure signal appears.
b) An additional error appears through "temperature influences" which influence the null-point displacement as well as the sensitivity of the measurement bridge.
c) A further error concerns the already-mentioned linearity error. In other words, the received pressure signal is not completely linear to the applied pressure. Especially in high precision situations, a correction must be undertaken here.
d) A further error is caused by the "sensitivity distributions" of the sensor cells, which are conditioned by production tolerances, as a result of which the individual cells display different sensitivities, for which reason the measurement circuit has to be appropriately balanced.
e) In addition, the so-called "hysteresis errors" and "repetitious errors" can also appear (these can only be detected with difficulty).
f) Finally, "long-term instabilities" appear, which basically are not detected at the beginning and become increasingly larger through long-term operation a nd contaminate the null-point.